A heterojunction solar cell is a solar cell that includes a heterojunction at an interface between two different semiconductors in the solar cell. A heterojunction solar cell can include a base layer, an intrinsic layer, and an emitter layer. FIG. 1 shows an example of a heterojunction solar cell 100 which includes a base crystalline silicon layer (c-Si) 102, an intrinsic amorphous silicon (a-Si) layer 104, a doped amorphous silicon (doped a-Si) layer 106, and a heterointerface 108 between semiconductor layers 102 and 104.
The base layer in a heterojunction solar cell can act as the primary light absorber. From this layer, photogenerated minority carriers can cross the heterointerface and the intrinsic layer before being collected as majority carriers in the emitter layer. The photogenerated minority carriers can cross the heterointerface either by tunneling, thermionic emission, or both depending on the heterojunction solar cell's operating point.
A band offset occurs at the heterointerface(s), such as heterointerface 108, in a heterojunction solar cell. At thermal equilibrium, Fermi-level constancy is maintained when the heterojunction solar cells undergo band bending at the heterointerface. For a fixed band offset, the strength of the band bending can depend on doping of the emitter layer, thickness of the intrinsic layer, as well as defects at the heterointerface. Strong band bending may assist in thermionic emission by lowering an effective barrier. Strong band bending may also assist in tunneling by decreasing effective barrier thickness. Additionally, strong band bending of the base layer surface can be associated with a strong electric field in the intrinsic layer and hence can assist in transporting minority carriers across the intrinsic layer.
Inversion of the heterointerface may also depend on the strength of the band bending, and fill factor can be influenced by inversion of the base layer. Fill factor is a measure of efficiency in heterojunction solar cells and is defined as the ratio of the maximum available power versus the product of the open circuit voltage and the short circuit current. A heterojunction solar cell with a high fill factor has a low series resistance and a high shunt resistance, which signifies smaller losses in current production internally. An increase in inversion of the base layer surface can result in a heterojunction solar cell with a high fill factor, whereas, a decrease in inversion of the base layer surface can result in a lower fill factor.
Manufacturing heterojunction solar cells with high fill factors is desirable. Unfortunately, however, determining that a heterojunction solar cell in the process of being manufactured has a high fill factor can be difficult. For example, in current techniques, a metalization layer typically needs to be applied to a solar cell so that fill-factor measurements on the solar cell can be made. The process of applying such a metalization layer can be time-consuming and expensive. Moreover, once the metalization layer has been applied, it is difficult to determine whether poor fill factor is due to the metalization lay or due to inherent properties of the solar cells.
Accordingly, new mechanisms for identifying heterojunction solar cells with high fill factors are desirable.